On the Calendar: Exoplanets and Worldships

byPaul GilsteronMay 27, 2011

Be aware of two meetings of relevance for interstellar studies, the first of which takes place today at the Massachusetts Institute of Technology. There, a symposium called The Next 40 Years of Exoplanets runs all day, with presentations from major figures in the field — you can see the agenda here. I bring this up because MIT Libraries is planning to stream the presentations, starting with Dave Charbonneau (Harvard University) at 0900 EST. Those of you who’ve been asking about Alpha Centauri planet hunts will be glad to hear that Debra Fischer (Yale University), who is running one of the three ongoing Centauri searches, will be speaking between 1130 and 1300 EST.

The poster for this meeting reminds me of the incessant argument about what constitutes a habitable planet. It shows two kids in a twilight setting pointing up at the sky, their silhouettes framed by fading light reflected off a lake. One of them is saying ‘That star has a planet like Earth.” An asterisk reveals the definition of Earth-like for the purposes of this meeting: “…a rocky planet in an Earth-like orbit about a sun-like star that has strong evidence for surface liquid water.” So this time around we really do mean ‘like the Earth,’ orbiting a G-class star and harboring temperatures not so different from what we’re accustomed to here.

And no, that doesn’t rule out more exotic definitions of habitability, including potential habitats around M-dwarfs or deep below the ice on objects far from their star. But finding an ‘Earth’ fitting the symposium’s definition would seize the public imagination and doubtless inspire many a career in science. The forty years referred to in the title of the meeting is a prediction that within that time-frame, we’ll be able to point to a star visible with the naked eye and know that such a planet orbits it. While following the events online, you might also want to track writer Lee Billings (@LeeBillings on Twitter), who’s in Cambridge for the day. Lee’s insights are invariably valuable.

Addendum: Geoff Marcy, amidst many a pointed comment about NASA’s priorities, also discussed in his morning session a probe to Alpha Centauri, as reported by Nature‘s newsblog:

On the back then of these serious policy criticisms came Marcy’s provocative idea for a mission to Alpha Centauri. He appealed to US President Barack Obama to announce the launch of a probe that would send back pictures of any planets, asteroids and comets in the system in the next few hundred years, with the US partnering with Japan, China, India and Europe to make it happen. “It would jolt NASA back to life,” he declared. Maverick it might sound, but many in the room seemed to take the idea in the spirit of focusing minds on the ultimate goal of planet-hunting; to take humanity’s first steps towards reaching out to life elsewhere in the universe.

Into the Generational Deep

The other conference, this one deep in the summer, takes us into the domain of far future technology. Some of the great science fiction about starships talks about voyages that last for centuries — I’m thinking not only of Heinlein’s Orphans of the Sky (1963, but drawing on two novellas in Astounding Science Fiction from 1941), but also Brian Aldiss’ Non-Stop (1958) and the recently published Hull Zero Three, by Greg Bear. In each case, we’re dealing with people aboard a vessel that is meant to survive for many human generations, a frequent issue being to identify just what is going on aboard the craft and what the actual destination is.

Call them ‘generation ships’ or ‘worldships,’ vast constructs that are built around the premise that flight to the stars will be long and slow, but that human technology will find a way to attain speeds of several percent of the speed of light to make manned journeys possible. A surprising amount of work has been done addressing the problems of building and maintaining a worldship, much of it appearing in the Journal of the British Interplanetary Society. Now the BIS is preparing a session dealing explicitly with worldships. Here’s their description of the event:

In 1984 JBIS published papers considering the design of a World ship. This is a very large vehicle many tens of kilometres in length and having a mass of millions of tons, moving at a fraction of a per cent of the speed of light and taking hundreds of years to millennia to complete its journey. It is a self-contained, self-sufficient ship carrying a crew that may number hundreds to thousands and may even contain an ocean, all directed towards an interstellar colonisation strategy. A symposium is being organised to discuss both old and new ideas in relation to the concept of a World Ship. This one day event is an attempt to reinvigorate thinking on this topic and to promote new ideas and will focus on the concepts, cause, cost, construction and engineering feasibility as well as sociological issues associated with the human crew. All presentations are to be written up for submission to a special issue of JBIS. Submissions relating to this topic or closely related themes are invited. Interested persons should submit a title and abstract to the Executive Secretary.

The session will run on August 17, 2011 from 0930 to 1630 UTC at the BIS headquarters on South Lambeth Road in London. The call for papers is available online. My own thinking is that a worldship is almost inevitable if we find no faster means of propulsion somewhere down the line. Imagine a future in which O’Neill-style space habitats begin to create a non-planetary choice for living and working. If we develop the infrastructure to make that happen, it’s not an unthinkable stretch to see generations that have adapted to this environment moving out between the stars.

Would their aim be colonisation of a remote stellar system? Perhaps, but my guess is that humans who have lived for a thousand years in a highly customized artificial environment may choose not to plant roots on the first habitable planet they find. They may explore it and study its system while deciding to stay aboard their familiar vessel, eventually casting off once again for the deep. In any case, worldships offer an interesting take on how we might make interstellar journeys relying not so much on startling breakthroughs in physics as steady progress in engineering and the production of energy. The London session should be provocative indeed.

Comments on this entry are closed.

andyMay 27, 2011, 11:35

Unfortunately it looks like Alpha Centauri has been dropped from the agenda… :-(

A naked eye visible G star with an Earth-like planet? Maybe. Certainly not a sure thing.

There are only about 60 potentially “visible” star systems containing at least one G star. Some of these are such luminaries as the G8 star HR7898, just 48 LY out, and dazzling with a visual magnitude of 6.36. Sharp eyed kids, get out the star chart!

As a practical point, I think it is safe to say that stars only known by HR number aren’t really visible. After all, Uranus is said to be visible (I have just barely seen it in very dark skies), but could not be discovered without a telescope.

Adding in F8 and F 9 stars doesn’t help much. Also, much of the sky is not visible from the Northern hemisphere, where most of the observers are. (eg: Alpha Centauri cannot be seen from the continental US, Europe, or Japan.) certainly there are fewer than 50 star systems containing even a broadly defined a Sol-like star actually visible from any particular site.

So what does that mean? For an Earth-like planet around a Sol-like star to be likely to be naked eye visible requires a >2% frequency of such planets around candidate stars. Certainly possible, in that such frequencies have not yet been disproven. However, there is little in the Kepler data so far to support such optimism.

Finally, if there is a planet in a visible star system meeting the MIT definition of Earth-like, it might fall far short of the Earth twins featured on Star Trek. A planet with liquid water on the surface sounds good for astrobiology. But what if it is a super-Earth with a surface gravity of 1.8g, and a world ocean 100km deep? Maybe ok for colonization by dolphins, but not much for Captain Kirk or the Robinson family to get excited about.

“Worldships” are essentially O’niell style space colonies with propulsion. This recent talk about rogue planets, and many more rogue comets and asteroids, suggests that interstellar migration may be more diffusion-like than that depicted in conventional SF. Unless we do get some kind of wormhole/FTL breakthrough, we will most certainly master the art of O’niell style space colonization before we go to the stars. Once people are used to that, they will probably stick with it rather than descend to the surface of any habitable planet that they find. One possibility is that they continue to live and do business in the habitat (and build many more of them) but use the planet for holiday and nature preserve.

Holger, don’t worry — Debra Fischer is continuing with her Alpha Centauri work. We were just talking about the fact that she did not appear at today’s conference, which could have been caused by any number of things.

Hmmm. I think you’ll find people willing to settle anywhere, including warm Neptune’s with 100km deep oceans and surface gravities of 0.8-1.2g, or gas dwarfs with 0.1-0.2g gravity at the top of a 1000 bar pressure atmospheres. Meanwhile, a lot more people will be inhabiting O’Neill cylinders, and Bernal spheres.

I’m not thinking many people will go in for the decadel journeys required for settling a “proper” star system – if we get up to 20-60& of c, I expect it to be used more for settling whatever lilypads are out there. All the batter for trading with… if there’s an average separation of, say, 3 light months between colonies, with high performance starcraft… perhaps a cycler ring will make sense; there’s your interstellar nomads.

Obviously, people who’ve been living in a worldship all their lives will, on arrival in a new planetary system, plant roots in the first habitable asteroid belt they find. From their point of view, so called habitable planets (i.e. surface water planets) are not part of their habitable zone at all.

They’ll need to build up a substantial local colony in order to create a local infrastructure large enough to fuel any further departures from that system.

As far as worldships, my guess is that the population will be divided between colonists who disembark and the rest who stay on board, with the latter likely being the majority. This was depicted on The Songs of Distant Earth by Arthur C Clarke.

Worldships might see themselves as more like mobile Cities and less like colonists traveling in search of a new home. Thus I suspect they’ll traverse the stars for reasons wholly apart from standard SF expectations. Zebrowski’s Macrolife star-travelers had the advantage of FTL drives, though they only developed those en route.

Star-Cities, as the Travelers might be dubbed, could see themselves as quite apart from the affairs of Worlds, as both Blish and Zebrowski depicted so memorably. Whether that’s a good thing or bad only time will tell. If suspended animation or relativistic flight prove viable this might all be academic, as small colonist groups could be sent instead of ponderous mini-planets.

With spring in full swing in North America, I notice some of my relatives having to cope with allergies brought on by the enormous proliferation of plants. Their sxs include stuffy nose, headache, sinus pressure, and even nausea at times. Being the space aficionado that I am I of course have to relate all the wheezing and sneezing to my primary interest.

Habitable planets may indeed exist that are very much like Earth with oceans and continents, photosynthetic unicellular and multicellular life, and the like. Future travelers to these hypothetical “alien Earths” could step off their spaceships and experience very familiar basic physical conditions/parameters (pressure, temperature, moisture content). However, what about the local biochemistry? I worry that other habitable planets with high biodiversity will flood future human travelers’ body systems with a myriad of antigenic and possibly very different and harmful substances that would put the aforementioned spring allergies to shame. Is this a genuine cause for concern? Might we be better off terraforming rather than settling worlds with preexisting life? Of course, it would be more exciting if bioincompatibility turned out not to be a show-stopper and my guess is that this issue will be determined on a case by case basis.

As for worldships, I find this option for interstellar travel the most likely one to occur (no offense to those hard at work on warp drive keep up the good creative work). As Paul cogently articulated: “worldships offer an interesting take on how we might make interstellar journeys relying not so much on startling breakthroughs in physics as steady progress in engineering and the production of energy.” Lastly, no offense to those hard at work on warp drive– keep up the good creative work!

Animals in an environment that goes periodically from abundant to extremely poor tend to enter a low energy consumption mode of some kind through the phase where feeding is not possible. If it has to be for a long time, then they periodically wake up for maintenance work. I think, that any species biotechnologically advanced enough to be capable of adapting its biology, will adopt this pattern. So even if they build large ships with self-sustained ecosystems, once they depart from a solar system and its sunlight and resources, they will hibernate, and wake up periodically only for a few days/weeks in a decade to make necessary repairs. So the ship will not be a true generation ship because the accumulated wake time even during a 1000 year journey, will be only a few years.
Also, I don’t think that they will find planetary surfaces somehow unattractive. Living in space means zero gravity most time, but also to be capable of withstanding high gees during maneuvers. So any species will most probably want to expand the acceleration range their bodies can withstand, not give it all up and become capable of living only in zero-g conditions. Therefore, I think, that at least a significant minority will decide to descend on a planet to live/work there. Even if it will not be a majority, there will be some. Even we humans frequently do choose to live in places that look totally different from where we were born. Moving from Canada to Costa Rica, or from sunny Spain to Island and stuff. From a small village to a big city and vice versa. I think that if an advanced species colonizes a new stellar system, they will leave no place empty.

I agree with Adam. There will always be far more energy efficient (i.e. much lower mass) methods of colonization. These methods will certainly achieve fractions of the speed of light and will be sent out in such large numbers in all directions that any slower worldship will find their target destination already colonized and possibly industrialized and paraterraformed.

However O’Neil cylinders will might eventually populate our and other star systems as legitimate alternate environs for people.

…that is of course if the Singularity doesn’t make all such thinking quaint.

Big kudos to Geoff Marcy for calling NASA and his fellow scientists out for showing a complete lack of leadership. The infighting among different TPF mission supporters is STILL going on too as a recent paper by a coronograph supporter had a single purpose: trash Wes Cash’s New Worlds external occulter concept. Marcy is exactly right that the New Decadal survey was a complete joke in that it ignored THE hottest topic in astronomy. Though Wes Traub has apparently been more “part of the problem rather than the solution” I think he’s exactly right in that a very good “pre-TPF” mission could be had for less than $1B; it really isn’t THAT hard and very much in line with Kepler. I do think that Marcy over-reached somewhat in what he hopes the President will do; simply making a TPF mission a NASA priority would be a very good start.
Surprised (well, not THAT surprised, as he is a Big Shot) that no one seriously got on D. Charbonneau’s case for calling for young scientists to ignore projects that take 15 years or so to finish. That would mean NO Kepler type missions AND NO next generation ground based telescopes.
What Charbonneau was really calling for was that everyone should duplicate HIS very successful career path.
Wes Traub seemed to come up with an eta-earth value that’s about 25x greater than that of most people; how did that happen?
I think Sara Seager should get into contact with the Bigelow folks; she could fly a superb photometric mission on one of the Bigelow habitats for less than what has already been invested in the ExoPlanetSat. That idea, by the way, is not very original, an Utah firm with expertise in Cube Sats has developed pretty much the same idea for low-cost earth imaging missions. Maybe this is what happens when a good theorist gets into the instrument development game (it is MUCH easier to motivate good undergrads if you give them a chance to build something though.) I think of ExoPlanetSat as a really good, expensive MIT outreach project that’s extremely unlikely to lead to any good science. Once you fly enough of them to be useful, the cost is up to what any of the all sky survey projects will cost, but with only a tiny fraction of the science return (since by design they’ll only find at most a bare handful of transiting earths, with no guarantee of finding ANY). I suspect this is the real reason she’s focussing her efforts on the commercial space flight industry, ExoPlanetSat has zero chance of passing peer review given the competition.

Agree with Frankenstein. Even in the 1960s I thought that Star Trek was crazy. Magical advances in physics with almost* zero advance in medicine or biotechnology. That mind set requires that FTL is needed as humans have no more than 50 years of vigorous adult life. (*Ok, Kahn was in a cryo storage on a slow ship, but human lifespan unchanged)

As I was originally a biologist, and have watched the physicists epic failures to control fusion the last several decades, my view is quite different. Biotechnology to allow life to exist in a dormant form for many thousands of years is much more achievable than near c travel and much more affordable in energy terms.

Likewise, as Frankenstein has noted, in theory there could be ETs who evolved the ability for extreme hibernation. This makes the Fermi paradox worse. Even 13,700 year interstellar voyages are only one millionth the age of the universe, essentially instantaneous in cosmic time. So, Where Are They?

Joy, an exobiologist, I don’t remember exactly who was it, said that there is, according to him, a possibility that communications between species that are deemed “underdeveloped”, or “immature”, from a technological/sociological/psychological/perhaps even biological aspect, by other, way more older, way more experienced, and way more advanced civilizations that are out there, and lurk “behind the scenes” so to speak, might actually be stopped/dampened/silenced? So as to avoid the creation of conflicts that might be common for species that are still quite in the “toddler” stage of evolution and progress (evaluated in their terms).

Let’s face it, we are not the most desirable company even for ourselves, as history bears witness of our own conflicts and wars that happened over the course of time, and this was our own species.

I may sound like Arthur C. Clark here, but maybe an alien race like the Firstborn in his books, to me, sounds quite plausible.

Like some sort of “cosmic sheppards”, keeping different flocks from intercepting each other until they become “ripe” so to speak.

Whoknows, maybe the universe is a lot more populated with intelligent life, but like I said, our means of communication, like the exobiologist also proposed, might be reduced to silence, by the cosmic equivalent of a “big brother”.

I think its something that would give a somewhat plausible answer to the Fermi Paradox.

Imagine in a couple of centuries, we achieve some type of (close to-) utopian society, and all of the sudden, our equipment picks up communications coming from all directions, of various forms and types.

You realize that somebody put the “radio” on “mute” all along.

Anyway, just my two cents here. Thought it would be an interesting idea to propose.

Latecomer to this discussion, but I agree with frankenstein: human descendants on board a colony ship, at least the most adventurous ones, will almost certainly want to settle a new found habitable planet, it would even be very hard stopping them from doing so! Even a thousand years is not nearly enough to change our human nature enough to take away that drive.

Furthermore I tend to believe that the future of interstellar travel and colonization, if no exotic FTL breakthrough propulsion is found, will consist of a combination of attaining a reasonable proportion of c plus a state of induced hibernation/suspended animation, in relatively small ships and during transition times as short as possible.

First of all, try to get enough qualified volunteers to start a generation ship, good luck (not me).
Secondy and probably more important, as I have argued before, the risks of catastrophic (internal) failure due to stochastic chance events (s..t happens) are simply enormous over the long run.
As previous discussions have made clear, this is not so much of a problem with a whole swarm of continuously manufactured and repaired space colonies in the vicinity of a star (light!) and asteroid like abundant resources.
And this is precisely how O’Neill like colonies were originally conceived.

However, during a centuries to millennia long journey through interstellar near-emptiness without refueling/restocking, repair-docking or emergency landing, the combination of loss of material and accidents/failures might prove too much of a risk. This is basically island-biogeography, the reason why small islands contain few species and even fewer large species.

Fermi’s Paradox may have a frighteningly simple answer: the daunting challenge to bridge the immense gaps between (suitable) stars.

Rather different from the SF novel. Does anyone know if the novel is available online anywhere? As old as this story is, it still packs a punch as I recall it. And of course it brings up the important question of whether or not a crew of “regular” humans could last in a civilized form on a worldship.

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last twelve years, this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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